Which type of mammal gives birth to fully developed live young?

Eutherians, also known as placental mammals, are characterized by giving birth to fully developed live young. This process involves a complex gestation period during which the embryo develops inside the mother's uterus, receiving nutrients and oxygen through the placenta. This allows eutherians to support the growth of their offspring more effectively compared to other types of mammals. Fully developed young are born, often capable of more immediate survival and independence than those of marsupials or monotremes.

Marsupials, on the other hand, give birth to relatively undeveloped young that typically continue to develop further in the mother's pouch. Monotremes are unique among mammals as they lay eggs rather than giving birth to live young. Species from the order Artiodactyla, which includes animals like deer and pigs, are primarily eutherians; however, the defining characteristic of eutherians is what sets them apart in this context—being specifically known for their fully developed live births. Thus, eutherians accurately represent the group that gives birth to completely formed young.